It is shown that the phase angle of the third harmonic of polarization response (delta3) is very sensitive to the ageing state of hard, Fe-doped lead zirconate titanate [Pb(Zr,Ti)O3 or PZT] ceramics and may thus provide rich information on processes responsible for ageing in hard ferroelectrics. When hard PZT ceramics experience hardening (aged state) - softening (deaged state) transition, delta3 changes by about 90 degrees at subswitching fields and by about 180 degrees at switching conditions. Evolution of delta3 with time, temperature and electric field amplitude during deageing suggests that at least two mechanisms of charge migration, one short-range and one long-range, may participate in the deageing process.
The dielectric response of hard (Fe-doped) and soft (Nb-doped) rhombohedral Pb(Zr0.58Ti0.42)1-xMexO3 (PZT; Me= Fe or Nb) ceramics was studied at subswitching conditions over a wide range of temperatures (50 C to 450 C) and frequencies (10 mHz to 10 kHz). The results show qualitative differences in behavior of the acceptor and donor doped samples. Hard materials exhibit a steep increase of the complex permittivity with decreasing frequency. The onset of the dispersion is thermally activated with activation energy of about 0.6-0.8 eV and is attributed here to oxygen vacancy hopping. Activation e…
Interactions among dislocations and solute atoms are the basis of several important processes in metals plasticity. In body-centered cubic (bcc) metals and alloys, low-temperature plastic flow is controlled by screw dislocation glide, which is known to take place by the nucleation and sideward relaxation of kink pairs across two consecutive \emph{Peierls} valleys. In alloys, dislocations and solutes affect each other's kinetics via long-range stress field coupling and short-range inelastic interactions. It is known that in certain substitutional bcc alloys a transition from solute softening to…
The Case Against Hall-Petch Hardening in High Entropy Carbide Ceramics
Missouri University of Science and Technology（密苏里科技大学）；Pennsylvania State University（宾夕法尼亚州立大学）；Duke University（杜克大学）；University of California, Irvine（加州大学尔湾分校）
Nanoindentation is a widely used method for sensitive exploration of the mechanical properties of micromechanical systems. We derived an empirical analysis technique to extract stress-strain field gradient and divergence representations from nanoindentation measurements. With this approach local gradients and heterogeneities can be discovered to obtain more detail about the sample's microstructure, thus enhancing the analytic capacity of the nanoindentation technique. We analysed nanoindentation tests of bulk solid substrates, namely bearing and tooling steel, and fused silica. Oscillations of…
A method for evaluating the piezoelectric response of unpoled ferroelectric ceramics from elastic and dielectric measurements is proposed and tested on BaTiO$_3$. The method is based on the observation that the softening in a ferroelectric phase with respect to the paraelectric phase is of piezoelectric origin. The angular averages of the piezoelectric softening in unpoled ceramics are calculated for ferroelectric phases of different symmetries. The expression of the orientational average with the piezoelectric and dielectric constants of single crystal tetragonal BaTiO$_3$ from the literature…
Many ceramic and hybrid metal-organic framework materials show negative thermal expansion (NTE): they \textit{contract} instead of expanding on heating \cite{Barrera_Miller_Lind_Romao 2005}. Their structures are invariably characterised as a network of polyhedral groups of atoms that are connected through sharing of corner atoms or by shared ligands. Empirically, NTE materials tend to show pressure-induced softening, pressure enhancement of NTE, and the reduction of NTE on heating. But such effects have only been investigated in a small number of materials \cite{Pantea 2006,Chapman 2005,Chapma…
Solid usually becomes harder and tougher under compression, and turns softer at elevated temperature. Recently, compression-induced softening and heating-induced hardening (CISHIH) dual anomaly was predicted in group VB elements such as vanadium. Here, the evidence for this counterintuitive phenomenon is reported. By using accurate high-temperature high-pressure sound velocities measured at Hugoniot states generated by shock-waves, together with first-principles calculations, we observe not only the prominent compression-induced sound velocity reduction, but also strong heating-induced sound v…
In this work, we address the issue of peaking of piezoelectric response at a particular composition in the morphotropic phase boundary (MPB) region of (Pb0.940Sr0.06)(ZrxTi1-x)O3 (PSZT) piezoelectric ceramics. We present results of synchrotron x-ray powder diffraction, dielectric, piezoelectric and sound velocity studies to critically examine the applicability of various models for the peaking of physical properties. It is shown that the models based on the concepts of phase coexistence, polarization rotation due to monoclinic structure, tricritical point and temperature dependent softening of…
Structural and Optical properties of the (Na0.5 K0.5)y Li 1-y NbO3 (y = 0.934 & 0.936) ceramics have been explored in terms of the X-Ray diffraction patters, Rietveld analysis, UV-Vis spectroscopy, and Kubelka-Munk functions. Rietveld analysis of both these structures reveals them to be tetragonal with space group P4mm. It is found that increase in Li doping increases the optical band gap. Presence of additional hardening dopant MnO2 is found to have a uniform behavior for both these composition. With increasing MnO2 concentration, the band gap decreases indicative of increase in B-site cation…
The softening and strengthening contributions in pre-deformed and aged Al-Mg-Cu alloys containing 3wt.%Mg and 0.5wt.%Cu are evaluated by a combination of microscopy, mechanical testing and modelling. A refined phenomenological model for the work hardening response, accounting for the separate effects of recovery and precipitation, is shown to be suitable for an unambiguous determination of the precipitation hardening contribution in these alloys. Significantly, it is found that the mechanical response of these alloys is not strongly impacted by Cu content (in the low Cu content regime), pre-de…
Effect of an internal chemical pressure on structural, optical, magnetic and electrical properties in Bi0.9RE0.1FeO3 Ceramics (RE =Gd3+,Tb3+,Dy3+)has been studied extensively. Some of the highlights are as follows; 1. Coexistence of orthorhombic Bragg reflections along with the R3c phase.(Structural Phase separation revealed from XRD and Raman spectroscopy) 2. Selective softening of phonon mode corresponding to oxygen motion suggests a significant destabilization in the FeO6 octahedra. 3. Three Charge transfer transitions (Fe1 3d\rightarrow Fe2 3d, Op \rightarrow Fed,Op \rightarrow Bip) and a …
We investigate the mechanical properties of amorphous polymers by means of coarse-grained simulations and nonaffine lattice dynamics theory. A small increase of polymer chain bending stiffness leads first to softening of the material, while hardening happens only upon further strengthening of the backbones. This nonmonotonic variation of the storage modulus $G'$ with bending stiffness is caused by a competition between additional resistance to deformation offered by stiffer backbones and decreased density of the material due to a necessary decrease in monomer-monomer coordination. This counter…
Niobium does not show any structure transition up to very high pressures. Nonetheless, by using density functional theory, we demonstrate in this work that it exhibits striking softening in elastic moduli C44 and C' at a pressure from 20 to 150 GPa. A novel anomaly softening in C44 from 275 to 400 GPa is also predicted. The physics behind these two anomalies is elaborated by electronic structure calculations, which revealed that they are actually different, with the first one directly relates to an underlying rhombohedral distortion whereas the latter originates in an electronic topological tr…
Molecular dynamics simulations are increasingly being used to investigate the structural evolution of polymers during mechanical deformation, but relatively few studies focus on the influence of boundary conditions on this evolution, in particular the dissipation of both heat and pressure through the periodic boundaries during deformation. The research herein explores how the tensile deformation of amorphous polyethylene, modelled with a united atom method potential, is influenced by heat and pressure dissipation. The stress-strain curves for the pressure dissipation cases (uniaxial tension) a…
On the thermal and mechanical properties of Mg$_{0.2}$Co$_{0.2}$Ni$_{0.2}$Cu$_{0.2}$Zn$_{0.2}$O across the high-entropy to entropy-stabilized transition
James Madison University（詹姆斯麦迪逊大学）；University of Virginia（弗吉尼亚大学）；Colorado School of Mines（科罗拉多矿业学院）；Laser Thermal Incorporated（激光热力学公司）；The Pennsylvania State University（宾夕法尼亚州立大学）
The latest experimental advances have extended the scenario of coupling mechanical degrees of freedom in chiral magnets (MnSi/MnGe) to the topologically nontrivial skyrmion crystal and even monopole lattices. Equipped with a spin-wave theory highlighting the topological features, we devise an interacting model for acoustic phonons and magnons to explain the experimental findings in a monopole lattice with a topological phase transition, i.e., annihilation of monopole-antimonopole pairs. We reproduce the anisotropic magnetoelastic modulations of elastic moduli: drastic ultrasonic softening arou…
